This does not necessarily mean that the ads are non-SSL (HTTP), simply that “something” is a different protocol—it could be unrelated to advertising. If the site is HTTP, it isn’t a big deal if some of the protocols are HTTPS (SSL); but if it’s the reverse, an HTTPS site won’t load HTTP calls.
Step 1: Prepare an email template detailing the recent changes your publisher deployed (i.e. switched from HTTP to HTTPS). Include a quick check if they support secure/HTTPS calls or not.
Step 2: Send out an email to each Ad Network actively running on your publisher’s site.
Step 3: Continue to monitor performance for any unusual dips. This could signal a demand partner not compatible with HTTPS.
Step 4: Work with your ad representative to address the issue.